1. Field of the Invention
2. Description of the Background Art
Semiconductor wafer processing equipment generally contains a vacuum chamber having a wafer support pedestal positioned within the vacuum chamber to support a wafer within the chamber atmosphere during processing. The pedestal generally contains a mechanism for retaining the wafer upon the pedestal surface. Such wafer retainers include mechanical clamps which physically retain the edge of the wafer and press the wafer against the pedestal surface, vacuum chucks that retain the wafer by establishing a vacuum beneath the wafer relative to the chamber pressure and electrostatic chucks that electrostatically retain the wafer in a stationary position upon the surface of the pedestal.
No matter what form of chuck is used to retain the wafer, it is important to determine when a semiconductor wafer has been positioned upon the pedestal and whether the wafer has been chucked (clamped) prior to processing the wafer. U.S. Pat. No. 5,436,790 issued Jul. 25, 1995 discloses a wafer presence and clamp condition monitoring apparatus. In this apparatus, a circuit monitors capacitance between two electrodes embedded within a wafer support pedestal. The capacitance falls into one range with no wafer positioned upon the support surface and into a second range with a wafer in place but not clamped. Furthermore, the capacitance falls in a third range with the wafer held in place by an electrostatic chuck formed when the embedded pair of electrodes are energized with a DC voltage. The monitoring circuit senses when the capacitance of the system is in each of the ranges by converting the measured capacitance to a DC voltage that can easily be sensed and used to confirm wafer placement and clamping.
Specifically, the electrostatic chuck used in the prior art system contains a pliable surface such that when the clamping force is applied, the wafer compresses the surface material and the wafer physically moves nearer to the pedestal surface and its embedded electrodes. This physical movement of the wafer relative to the electrodes causes a change in the capacitance between the electrodes. Such pliable surface materials are only useful in low temperature semiconductor processing systems. At high temperatures, these materials breakdown, outgas and/or deform. As such, at high temperatures, the electrostatic chuck having a pliable surface would contaminate the chamber.
Ceramic electrostatic chucks that are typically used in high temperature semiconductor wafer processing are constructed of a ceramic material that becomes somewhat conductive at high temperatures (i.e., the resistivity of the material decreases with increased temperature). Specifically, when the wafer rests flush against the surface of the chuck body while chucking voltage is applied one or more embedded electrodes, the wafer is primarily retained against the ceramic support by the Johnsen-Rahbek effect. One example of such a ceramic chuck is disclosed in U.S. Pat. No. 5,117,121 issued May 26, 1992 and incorporated herein by reference.
Ceramic chucks of this type have a hard, non-pliable surface that does not break down or deform during high temperature wafer processing. As such, the capacitive wafer position monitoring systems such as that described above have not heretofore been used in conjunction with these chucks because the surface is not pliable to allow the wafer to move substantially closer to the electrodes when chucked.
Therefore, a need exists in the art for a system that detects wafer presence and operates at high wafer processing temperatures.
The disadvantages heretofore associated with the prior art are overcome by the present invention of an apparatus and method for detecting the presence of a wafer as the wafer is positioned upon a non-pliable surface of a high-temperature semiconductor wafer support pedestal, e.g., a ceramic electrostatic chuck. The invention also detects whether a wafer positioned on the support surface has been damaged, whether the wafer has been chucked and whether a wafer has a positive or negative bow.
The first embodiment of the invention contains a plurality of electrodes affixed to the surface of the wafer support pedestal and arranged such that all the electrodes are covered by a wafer that is properly centered upon the pedestal surface. Specifically, the electrodes are arranged in three pairs, where each of the pairs is equilaterally positioned proximate the edge of the pedestal surface. The electrodes are coupled to a capacitance measurement circuit that measures the capacitance between the electrodes of each pair and generates a signal corresponding to a wafer""s presence, location and chucking condition. Wafer presence increases the capacitance between the electrodes by providing substantial capacitive coupling between the electrodes. Increasing chucking force also increases capacitance by decreasing the gap between the wafer and the chuck surface. This gap capacitance can be utilized to detect wafer sticking after dechucking or to adjust a chucking voltage to obtain a pre-determined chucking force.
In a second embodiment of the invention, a plurality of electrodes are unpaired and spaced equilaterally about the circumference of the wafer support pedestal (e.g., three surface electrodes). A common electrode (e.g., a fourth surface electrode) is positioned near a center of the wafer support pedestal. The presence of the wafer on the support surface completes a conductive path between the electrodes, i.e., the wafer contacts all four of the surface electrodes. Any damage to the wafer, such as cracks in the wafer, a broken wafer, and the like, will interrupt the conductive path or paths. As such, the second embodiment of the invention enables damaged wafers to be detected as well as detect wafer presence. Furthermore, if the wafer is not centered on the pedestal surface and one of the electrodes is not covered by the wafer, the conductive path is not complete with respect to that electrode. As such, the invention can detect when a wafer is improperly positioned on the support surface.
A third embodiment of the invention comprises a plurality of surface edge electrodes disposed about the circumference of a chuck surface. These electrodes, which are interconnected with one another, form an outer surface electrode. A circumferential conductor that is deposited upon a vertical, circumferential edge of a support region of the chuck provides the interconnection amongst all the edge electrodes to form a single, unified outer surface electrode. An inner surface electrode is disposed proximate the center of the wafer support surface of the chuck. In a simple form, the capacitance between the inner and outer surface electrodes is measured to identify the presence of a wafer, whether the wafer is chucked, and whether the wafer is centered in much the same manner as the capacitance is measured between the electrodes of the second embodiment of the invention.
However, to promote additional measurement accuracy and flexibility, the capacitance is preferably measured between an embedded electrode or electrodes of the electrostatic chuck and the inner and outer surface electrodes, respectively. As such, wafer presence, position, and chucking condition is determined through these capacitance measurements. Additionally, the wafer bow orientation is also determined when the wafer is first positioned upon the chuck support surface. For example, when a negatively bowed wafer is placed upon the chuck surface, the wafer contacts the inner surface electrode, but not the outer surface electrode. As such the capacitance measured between the inner surface electrode and the embedded electrode(s) is much higher than the capacitance measured between the outer surface electrode and the embedded electrode (i.e., for a wafer having a negative bow, the inner electrode contacts the wafer and the outer electrode does not contact the wafer). The opposite capacitance measurement is found when the wafer is positively bowed (i.e., for a wafer having a positive bow, the inner electrode does not contact the wafer and the outer electrode does contact the wafer). Thereafter, the chucking voltage can be incrementally applied to the embedded electrodes until the wafer contacts both electrodes, indicating a xe2x80x9cchuckedxe2x80x9d condition.
As a result of using the present invention, the presence of a wafer, the position of the wafer, the direction of wafer bow, and the condition of the wafer, i.e., wafer damage, can be detected upon a wafer support pedestal during wafer processing. Furthermore, the invention can be used in conjunction with any non-pliable pedestal surface.